One known form of apparatus for filtering, washing and cooling gases comprises an inlet duct for conducting such gas to a chamber, an outlet duct for conducting gas away from this chamber, a foraminous wall extending across the chamber and through which gas passes on its way from the inlet duct to the outlet duct, means for forcing gas through the foraminous wall and a liquid sprayer for spraying a curtain of water across the chamber between the inlet duct and the wall. Various liquid sprayers may be used to produce a spray or mist of liquid droplets in such apparatus. Typically, in such liquid sprayers a rotor having the form of a cylinder or disc dips just below the surface of a bath of liquid in a container. The rotor revolves at high speed so that it picks up liquid during its passage therethrough and then flings the liquid tangentially away from the rotor. In designing such sprayers, it is desirable that the rotor be made to lift as much liquid as possible from the bath of liquid into which it dips, but it is also necessary to ensure that the spray or mist of droplets which is flung from the rotor is as uniform as possible, both as regards the sizes of the droplets therein and their dispersion within the curtain of spray formed by the rotor.
Three types of rotor are known in the art, namely smooth cylinders, smooth discs, and toothed discs, that is to say discs bearing projections extending out of the plane of the discs. Smooth cylinders are bulky and so heavy that they are difficult to balance on a shaft revolving at high speed. In addition, since the water or other liquid being sprayed only adheres to the surface of the cylinder by surface tension, the quantity of water sprayed by a smooth cylinder is small relative to the large power needed to rotate a heavy cylinder at high speed. Smooth discs are lighter and easier to balance than smooth cylinders but spray only a small amount of water so that a very large number of discs may be necessary where large quantities of water have to be sprayed. Toothed discs spray far more water than smooth discs because the projections act as scoops lifting water out of the bath. Unfortunately, the spray produced by known apparatus using toothed discs lacks uniformity, both as regards to the sizes of individual droplets within the spray and the angular dispersion of the spray.